Abstract Rats were injected with 10 5 cells of Walker carcinosarcoma 256 in ascites form into discrete areas of the brain (RAS, midbrain‐hypothalamus, and anterior and posterior cortex), and at necropsy on the sixth day postoperation the animals were sacrificed, the brains were examined for tumors, and liver homogenates were analyzed for catalase content. The latter was significantly depressed in the tumor‐bearing animals as compared to the corresponding saline‐injected controls. The findings are of interest considering that the tumors were relatively small; however, they occupied significant areas of the entire brain. Although small or moderate body weight losses were encountered, the inhibitory effect was not due to inanition, a known depressant. Liver catalase was little altered in rats with solid tumors in the various brain areas originating from fragments measuring 1 mm 3 and harvested from donors with intramuscular Walker tumor, as was also the case with tumor‐free rats submitted to hypothalamic lesioning. Brain catalase levels were not affected by any of the treatments.